2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 204-10
Presentation Time: 9:00 AM-6:30 PM


CAMPBELL, Mary K., Geology, Oberlin College, 173 W Lorain St., Oberlin, OH 44074, BOWER, Jennifer A., Geology, The University of Vermont, The University of Vermont, Burlington, VT 05405, SCHMIDT, Amanda H., Geology, Oberlin College, Geology Department, Rm. 416, 52 W. Lorain St, Oberlin, OH 44074, NEILSON, Thomas B., Department of Geology, University of Vermont, 180 Colchester Ave, Department of Geology, UVM, Burlington, VT 05401, SOSA-GONZALEZ, Veronica, The Rubenstein School, University of Vermont, Aiken Center, 81 Carrigan Drive, Burlington, VT 05405 and BIERMAN, Paul R., Department of Geology and Rubenstein School of Environment and Natural Resources, University of Vermont, Delehanty Hall, 180 Colchester Ave, Burlington, VT 05405, mcampbel@oberlin.edu

Between 1950 and 1980, many areas of southwest China were widely deforested but sediment yield data show no significant variation over that time, posing the question: Did land use changes accelerate erosion? We aim to understand how land use history affects erosional processes by using 210Pbex and 137Cs to unravel possible land use histories and comparing the activities of end-member soil profiles with downstream detrital samples.

Soil profiles were sampled from three tributary basins to the Mekong River with areas ranging from 198 to 2508 km2 and mean annual precipitation from 869 to 1299 mm/yr. Land use in the basins encompasses forest, agricultural land, and grassland. We collected seven soil profile samples (five in one basin, one in each of the others) in 5 cm increments to 30 cm depth, and took samples of active channel sediment downstream of these pits. We then compared the activities of fallout radionuclides in the sediment to the soil profile samples to determine what proportion of active channel sediment is sourced from areas represented by the soil pit sample sites.

The soil profiles demonstrate a wide range in radioisotope concentrations with depth and between sites (surficial measurements of 210Pbex ranges from 45-152 Bq/kg and 137Cs ranges from 7-339 Bq/kg). Several profiles demonstrate differing rates in the decay of radionuclide concentrations with depth, suggesting a change in erosion rates. Other profiles demonstrate a complete absence of 210Pbex and 137Cs below 10-20cm, indicating extensive erosion.

The connection in radioisotope concentrations between detrital samples and upstream soil pits is less well constrained. Preliminary results show a potential dilution effect in radionuclide concentrations in detrital samples moving downstream from source sites. At one sample site, 137Cs and 210Pbex activities are much lower in detrital samples than in the soil pits, going from 152 to 7 Bq/kg and 223 to 4 Bq/kg, respectively. This dilution effect appears to be amplified in detrital Cs measurements, to the extent that the majority of Cs readings fell below detection limits. Ultimately, our ability to correlate detrital samples with end member sources may be limited by complexities in sediment transport in fluvial systems.